The invention concerns a two-stage valve, in particular a brake valve.
It is known to provide a two-stage brake valve which includes a first stage with a larger effective area used initially to fill a brake system component and a second stage with a smaller effective area used in succession following a transition phase to generate an operating pressure. It is also known to provide such a two-stage brake valve with a check valve arranged between the pressure chamber of the first stage add a reservoir, which opens upon a pre-determined brake pressure existing in the first stage pressure chamber.
A two-stage brake valve of the aforementioned type has become known from DE-A-1 680 211, which includes a fill piston with a large effective area. The fill piston slides and is sealed in a bore in the valve housing and can be moved in the bore by a piston rod. The piston rod can be actuated by an operator through a brake pedal and associated linkages. The fill piston pressurizes fluid in a first pressure chamber. The fill piston contains an axial bore open to its end face, which receives a hollow sleeve fixed to the valve body but which can move within the bore. The axial bore in the fill piston forms a second pressure chamber with a smaller effective area. The sleeve is provided with a central bore through which the second pressure chamber is connected to the brake line. The first pressure chamber is connected to a reservoir through a return line containing a check valve. The check valve opens when a predetermined pressure in the first pressure chamber is exceeded.
Between the second pressure chamber and the first pressure chamber a valve arrangement is located which initially permits an exchange of fluid between the pressure chambers and then seals these off against each other only after a predetermined pressure difference between the pressure chambers is exceeded. During a first phase of movement of the fill piston the brake chamber of the brake is filled until the pressure in the first pressure chamber increases to the point at which the check valve opens. Upon further actuation of the fill piston the pressure in the second pressure chamber increases further and leads to a closing of the valve arrangement between the pressure chambers, so that in a pressure phase only the smaller effective area of the shank is used to generate the operating pressure, and a high braking pressure can be applied with comparatively lower actuating force. However, during the pressure phase there exists in the first pressure chamber a continuing pressure corresponding to the opening pressure of the check valve. Therefore the operator must apply a force in addition to that required to generate the operating pressure which corresponds to the opening pressure in the first pressure chamber.
In order to reduce the required pressure during the pressure phase, a brake valve was proposed by DE-A-38 37 650, which is configured similar to the valve described above, in which, however, the first pressure chamber is constantly connected directly to the reservoir during the pressure phase. However, this leads to a sudden pressure drop in the pressure chamber during the transition from the fill phase to the pressure phase.
In the case of the last named brake valve, it has been shown that upon the transition from the larger effective area to the smaller effective area, in which a connection between the first pressure chamber and the reservoir is suddenly opened, the pressure in the second pressure chamber is at a nearly constant pressure level for the time of the transition, and therefore the required pedal force is suddenly reduced. The operator, with a limited reaction time, cannot adjust rapidly enough to this sudden change in the force requirement. As a result the operator will depress the brake pedal with a constant force, as was required for the larger effective area during the filling of the brake, also during the second pressure phase in which only the smaller effective area is used. If the actuating force is held constant, a higher pressure will be generated with the smaller effective area as compared to the larger effective area. A substantial increase in the pressure in the line to the brake cylinder (actuating device) results in a sudden increase in the braking force and therewith a sudden increase in the deceleration of the vehicle. This means that the relationship between the pedal force and deceleration is not proportional or follows some other transition, but experiences a sudden jump, which is not permissible or at least not desired in vehicle brakes.